[Tock Devel] Type-safe syscall exploration

Inspired by the ongoing discussions about porting Tock to CHERI, I wrote down some ideas on how to make Tock's syscall ABI more type-safe. The writeup is in the design-explorations repository at Add an exploration of type-safe system calls <https://github.com/tock/design-explorations/pull/4>. While I'm not suggesting that we should start on Tock 3.0 now, I do think we should incorporate some of the design ideas into Tock 3.0 when it happens. A secondary goal of the document is to spread knowledge about CHERI. Feel free to take a look, and either reply with feedback here or comment on the PR. -Johnathan